1. Field of the Invention
The present invention relates to a process for signaling an interruption in the state of readiness for reception of a data terminal in a called station, in data transmission over a data selector network, in particular for teleprinters, wherein following the establishment of the connection the calling station requests, via the data selector network, the called station to transmit its identification, and following the reception of the identification assigned to the called station, the data transmission begins.
2. Description of the Prior Art
In data transmission employing teleprinters which transmit and receive data by way of the Telex network in accordance with the start-stop principle, the start-stop teleprinters are constantly in a state or readiness for reception. Following the establishment of the connection by the calling station, the assigned identifications are exchanged between the two stations, and the same are printed out on the respective teleprinter. The continuous state of readiness for receiving data ensures that the transmitted item of data is actually printed out or recorded in the data terminal device of the called station, it being immaterial whether the receiving teleprinter is attended or unattended.
However, the state of continuous readiness for reception of a start-stop teleprinter can become disturbed or impaired when, for example, the paper in the page printer or the punched tape comes to an end, when the ribbon is exchanged, or the paper is being inserted into the page printer, and therefore the housing is removed and the housing cover opened. Although the calling, transmitting station and the called station have successfully effected the connection, and the identification exchange, in the event of a disturbance in the state of readiness for reception, the item of data is lost, either partially or entirely.
It is already well known in start-stop teleprinters to provide a monitoring circuit, for example in the form of sensing contacts, which serves to monitor the end of the paper in the printer or the punched tape feed unit, and the opening and closing of the housing cover. When the monitoring device responds, the connection is cleared. This results in an interruption in the teleprinter transmission. In this case, the calling station is not aware, however, of the discontinuation of the state of readiness for operation.
A process is already known for signaling an interruption in the state of readiness in the reception in electronic teleprinters, wherein following the response of the data carrier end contact, an existing Telex connection is cleared in the printer of the teleprinter. In a situation in which a call is incoming, and the data carrier simultaneously comes to an end, and the housing cover is open, the remote switching device signals to the subscriber connection line the state of "subscriber disturbed" for a teleprinter single terminal, for example by continuous Z-polarity, and the state "subscriber engaged" in the case of the teleprinter collective terminal, for example by a continuous A-polarity. However, the last-mentioned known signaling technique frequently does not exist in the Telex network or in other data networks, since the devices, for example switching systems, do not permit these procedures, or because the requisite switching devices are simply not available.